Method for fabricating a structure for a microelectromechanical systems (MEMS) device

ABSTRACT

The invention provides a microfabrication process which may be used to manufacture a MEMS device. The process comprises depositing one or a stack of layers on a base layer, said one layer or an uppermost layer in said stack of layers being a sacrificial layer; patterning said one or a stack of layers to provide at least one aperture therethrough through which said base layer is exposed; depositing a photosensitive layer over said one or a stack of layers; and passing light through said at least one aperture to expose said photosensitive layer.

FIELD OF THE INVENTION

This invention relates to microfabrication. In particular, it relates tothe microfabrication of a structure for a Microelectromechanical Systems(MEMS) device.

BACKGROUND

Microfabrication techniques used to fabricate MEMS devices generallyinvolve the deposition of one or more layers on a substrate and thesubsequent patterning of the layers to produce useful structures. Onetechnique for patterning a layer involves the use of photolithography.With photolithography a photographic definition of a desired pattern ona photo or optical mask is used to impart the pattern onto a surface ofthe layer. When manufacturing a MEMS device usually several maskingsteps are required, each masking step adding to the cost of the device.Accordingly, it is desirable to reduce the number of masking stepsrequired during fabrication of a MEMS device.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided amicrofabrication process comprising depositing a first layer on asubstrate; patterning the first layer; depositing a second layer on thefirst layer; and patterning the second layer using the first layer as aphotomask.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a three dimensional drawing of a part of a MEMS devicewhich may be manufactured using the microfabrication process of thepresent invention; and

FIGS. 2 to 10 shows various stages in the manufacture of the MEMS deviceof FIG. 1.

DETAILED DESCRIPTION

Aspects of the present invention will now be described with reference toFIGS. 2 to 10 of the drawings which show the stages during fabricationof a MEMS device such as a Visible Spectrum Modulator Array described inU.S. Pat. No. 5,835,255 or an Interferometric Modulater (IMOD) describedin U.S. Pat. No. 6,040,937. Naturally, describing the present inventionwith reference to stages in the manufacture of a Visible SpectrumModulator Array or an IMOD is intended to enhance understanding of thepresent invention and it is to be understood that the present inventionmay used in the manufacture of other MEMS devices. Thus, the descriptionof the present invention with reference to the manufacture of a VisibleSpectrum Modulator Array or an IMOD is intended to be non-limiting.

FIG. 1 of the drawings shows an example of a part of a Visible SpectrumModulator Array 10 which may be fabricated in accordance with techniquesdescribed herein. Referring to FIG. 1, an antenna array is fabricated onone-half of a microfabricated interferometric cavity which transmits andreflects certain portions of incident electromagnetic radiationdepending on (a) the dimensions of the cavity itself and (b) thefrequency of response of dielectric mirrors in the cavities. In FIG. 1,the array 10 is shown to include two cavities 12, 14 fabricated on atransparent substrate 16. A layer 18, the primary mirror/conductor maycomprise a combination of one or more films of metals, oxides,semiconductors, and transparent conductors. Insulating supports 20 holdup a second transparent conducting membrane 22. Each array element hasan antenna array 24 formed on the membrane 22. The two structures 22,24, together comprise the secondary mirror/conductor. Conversely, theantenna array may be fabricated as part of the primary mirror/conductor.Secondary, mirror/conductor 22/24 forms a flexible membrane, fabricatedsuch that it is under tensile stress and thus parallel to the substrate,in an undriven state.

Because layers 22 and 24 are parallel, radiation which enters any of thecavities from above or below the array can undergo multiple reflectionswithin the cavity, resulting in optical interference. Depending on thedimensions of the antenna array, the interference will determine itsreflective and/or transmissive characteristics. Changing one of thedimensions, in this case the cavity height (i.e. the spacing between theinner walls of layers 18, 22), will alter the optical characteristics.The change in height is achieved by applying a voltage across the twolayers of the cavity, which due to electrostatic forces, causes layer 22to collapse. Cavity 12 is shown collapsed (7 volts applied), whilecavity 14 is shown uncollapsed (0 volts applied).

In fabricating the array 10, it is desirable that insulating supports 20are well defined in regions where contact is made between insulatingsupports 20 and layers 18, 22. The present invention is especiallyuseful in manufacturing such a support. FIGS. 2 through 10 show variousstages in the manufacture of a MEMS device having supports such as thesupports 20. Referring to FIG. 2 of the drawings, reference numeral 100indicates a substrate 100. The substrate 100 may be of many differentmaterials each being transparent to ultraviolet light. Examples of thesematerials include plastic, mylar, or quartz. The material must be ableto support an optically smooth, though not necessarily flat, finish. Apreferred material would likely be glass, which would be bothtransmissive and reflective operation in the visible range.

Various layers are deposited on substrate 100 to define a stack. Inparticular, the substrate 100 is coated with a sacrificial layer 102using standard techniques such as a physical vapor deposition (PVD)method such as sputtering or e-beam evaporation. Other possible methodsinclude chemical vapor deposition and molecular beam epitaxy.

In FIG. 2, the sacrificial layer is a single layer. However, in otherembodiments of the invention layer 102 may be a stack of layers with anuppermost sacrificial layer.

FIG. 3 of the drawings shows a stage in the manufacture of the MEMSdevice wherein the sacrificial layer 102 has been patterned so as todefine longitudinally spaced grooves 104. A standard procedure is usedto pattern sacrificial layer 102 which includes exposing layer 102through an appropriate mask and developing to produce the pattern.

In FIG. 4 of the drawings, a photosensitive polymeric material in theform of a negative-acting-photosensitive material which could be anegative photoresist has been spun onto sacrificial layer 102 with athickness that is larger than the total height of the film stack definedby layers 100 and 102. Thereafter, the negative-acting-photosensitivematerial is exposed to ultraviolet light through substrate 100 anddeveloped using conventional techniques. Because the longitudinalgrooves 104 are the only means by which thenegative-acting-photosensitive material is exposed, thenegative-acting-photosensitive material over the stack is dissolvedduring a subsequent development process, leaving only longitudinalridges 106 of negative-acting-photosensitive material disposed ingrooves 104. Thus, it will be appreciated that by first patterning thesacrificial layer 102 and then exposing thenegative-acting-photosensitive material through substrate 100 throughlongitudinal grooves 104 in the sacrificial layer 102, the sacrificiallayer 102 acts as a photomask thereby allowing thenegative-acting-photosensitive material to be lithographically patternedwithout the need for an additional masking step. In FIG. 5 of thedrawings, a structural layer 108 has been deposited on the stack and thesacrificial layer 102 has been removed, thus the layer 108 is supportedby ridges 106. It will be appreciated that by using different photomasksit will be possible to fabricate support structures of any desiredgeometry. Thus instead of ridges, in other embodiments pillars or postsmay be formed. The layer 108 is highly conductive and reflective andwill typically contain aluminum and nickel.

FIG. 6 of the drawings shows a subsequent stage in the manufacture ofthe MEMS device wherein the layer 108 is patterned into transverselyextending strips. FIG. 7 of the drawings shows the film stack with anoxide spacer layer 110 deposited on layer 108. FIG. 8 of the drawingsshows a stage in the manufacture of the MEMS device in which the oxidespacer layer 110 has been patterned. FIG. 9 of the drawings shows astage in the manufacture of the MEMS device in which a sealing film 112is being applied with a pressure adhesive 114 over the entire structureto protect the structure from damage due to mechanical shock loading andto prevent particulates from interfering with the operation of the IMODstructures. The sealing film 112 could be of a variety of materials suchas thin metal films or polymeric films which have been coated with ametal or oxide film to provide hermeticity. Finally, FIG. 10 shows thestructure after it has been purged with XeF₂ gas to remove the remainsof sacrificial layer 102. The edges of the structure are then sealed.

In other embodiments, instead of oxide layer 110 another layer ofnegative-acting-photosensitive material may be spun over oxide layer 110and exposed through substrate 100 and using the techniques describedabove a further set of support structures may be formed. These supportstructures will provide support for other layers. It will be appreciatedthat the process may be repeated to build a MEMS structure havingmultiple layers or “floors” stacked one on top of the other wherein thefloors are vertically spaced by support structures fabricated inaccordance with the above techniques. One advantage of the presentinvention is that it provides a microfabrication technique which allowsa mechanical support between two layers in an MEMS device to beprecisely defined. This allows a clean, well-defined mechanical contactbetween the support and other structures within the MEMS device.

Further, the present invention uses a patterned layer on a substrate asa photomask to pattern another layer, thereby saving on a masking step.

Although the present invention has been described with reference tospecific exemplary embodiments, it will be evident that the variousmodification and changes can be made to these embodiments withoutdeparting from the broader spirit of the invention as set forth in theclaims. Accordingly, the specification and drawings are to be regardedin an illustrative sense rather than in a restrictive sense.

What is claimed is:
 1. A microfabrication process for fabricating amicroelectromechanical systems device, comprising: depositing one or astack of layers on a substrate; patterning said one or a stack of layersto form a plurality of longitudinally extending groves therein;depositing a middle layer on said one or a stack of layers; patterningthe middle layer using said one or a stack of layers as a photomask,wherein said middle layer is exposed to light passed through the grovesin the one or a stack of layers; and developing said middle layer toform longitudinally spaced ridges in the said middle layer disposed inthe groves in said one or a stack of layers, said ridges forming asupport structure which is part of the microelectromechanical systemsdevice.
 2. The method of claim 1, wherein the substrate permits light topass therethrough.
 3. The method of claim 2, wherein the substratecomprises glass.
 4. The method of claim 1, further comprising depositinga top layer over said middle layer.
 5. The method of claim 4, whereinsaid top layer comprises nickel and aluminum.
 6. The method of claim 4,further comprising patterning said top layer.
 7. The method of claim 6,wherein said top layer is patterned to define transversely extendingstrips which are supported by the longitudinally spaced ridges in thesaid middle layer.
 8. The method of claim 1, wherein said one or anuppermost layer of said stack of layers is a sacrificial layer.
 9. Themethod of claim 1, wherein the said middle layer comprises anegative-acting-photosensitive material.
 10. A method for fabricating amicroelectromechanical systems device, the method comprising: a)depositing one or a stack of layers on a base layer, said one layer oran uppermost layer in said stack of layers being a sacrificial layer; b)patterning said one or a stack of layers to provide at least oneaperture therethrough through which said base layer is exposed; c)depositing a photosensitive layer over said one or a stack of layers; d)passing light through said at least one aperture to expose saidphotosensitive layer; and e) developing said one or a stack of layers toremove unexposed portions of said photosensitive layer and saidsacrificial layer to form at least one mechanical support, each supportbeing defined by a portion of said photosensitive layer exposed throughan aperture.
 11. The method of claim 10, wherein the base layer is asubstrate layer.
 12. The method of claim 10, wherein said lightcomprises ultraviolet light.
 13. The method of claim 10, wherein saidphotosensitive layer comprises a negative-acting-photosensitivematerial.
 14. The method of claim 10, further comprising depositing astructural layer over said photosensitive layer.
 15. The method of claim14, wherein said steps (a) to (e) are repeated at least once, whereinfor each repetition, the structural layer forms the base layer.